The healthy heart produces regular, synchronized contractions. Rhythmic contractions of the heart are normally controlled by the sinoatrial (SA) node, which is a group of specialized cells located in the upper right atrium. The SA node is the normal pacemaker of the heart, typically initiating 60-100 heartbeats per minute. When the SA node is pacing the heart normally, the heart is said to be in normal sinus rhythm.
If the heart's electrical activity becomes uncoordinated or irregular, the heart is denoted to be arrhythmic. Cardiac arrhythmia impairs cardiac efficiency and may be a potential life-threatening event. Cardiac arrhythmias have a number of etiological sources, including tissue damage due to myocardial infarction, infection, or degradation of the heart's ability to generate or synchronize the electrical impulses that coordinate contractions.
Bradycardia occurs when the heart rhythm is too slow. This condition may be caused, for example, by impaired function of the SA node, denoted sick sinus syndrome, or by delayed propagation or blockage of the electrical impulse between the atria and ventricles. Bradycardia produces a heart rate that is too slow to maintain adequate circulation.
When the heart rate is too rapid, the condition is denoted tachycardia. Tachycardia may have its origin in either the atria or the ventricles. Tachycardias occurring in the atria of the heart, for example, include atrial fibrillation and atrial flutter. Both conditions are characterized by rapid contractions of the atria. Besides being hemodynamically inefficient, the rapid contractions of the atria may also adversely affect the ventricular rate.
Ventricular tachycardia occurs, for example, when electrical activity arises in the ventricular myocardium at a rate more rapid than the normal sinus rhythm. Ventricular tachycardia may quickly degenerate into ventricular fibrillation. Ventricular fibrillation is a condition denoted by extremely rapid, uncoordinated electrical activity within the ventricular tissue. The rapid and erratic excitation of the ventricular tissue prevents synchronized contractions and impairs the heart's ability to effectively pump blood to the body, which is a fatal condition unless the heart is returned to sinus rhythm within a few minutes. Ventricular fibrillation, for example, may be treated by delivering an electrical shock to the patient's heart through the use of a defibrillator.
Cardiopulmonary resuscitation (CPR) is commonly used to maintain life support for victims of cardiac arrest until a defibrillator can be deployed to treat the arrhythmia. In the absence of defibrillation therapy, the chances of surviving a cardiac arrest decrease rapidly as the duration of time following the arrest increases. Quick response to a cardiac arrest by performing CPR and subsequently administering a defibrillating shock is therefore of critical importance. The American Heart Association's “Chain of Survival,” for example, recites the following steps:                1. Early access to emergency care, such as by activating an emergency medical system (EMS);        2. Early CPR initiated by a bystander or other first responder using basic life support (BLS) techniques to help the patient survive until more advanced care arrives;        3. Early defibrillation; and        4. Early advanced cardiac care.The benefits of this approach are discussed in more detail in Cummins, et al. “Improving Survival From Sudden Cardiac Arrest: the ‘Chain of Survival’ Concept,” 83 Circulation 1832-47 (May 1991).        
EMS providers are playing an active role in implementing the Chain of Survival concept. Tiered EMS systems are emerging in many geographical areas and are typically divided between first responders, BLS providers, and ACLS (advanced cardiac life support) providers. First responders and BLS providers, often called EMT(B) or EMT-basic (emergency medical technician-basic), represent front line personnel who are first to reach a patient. Such personnel are now being trained and authorized to use automatic external defibrillators (AEDs) to provide early defibrillation.
Until the EMS provider arrives, CPR should be administered. Many people have been trained in CPR techniques, and are capable of performing CPR. However, although trained, many people never practice the techniques after initial training, and may be unsure of appropriate responses in an emergency, before the EMS provider arrives.
The compression during a CPR provides an external mechanical stimulation to the heart. Coordinating this mechanical stimulation with internal electrical stimulation of the heart may be beneficial to re-establishing normal sinus rhythm during a cardiac arrest.